Heart disease and heart failure remain the leading causes of morbidity and mortality worldwide. Adult mammalian heart demonstrates limited regenerative potential. Numerous measures, such as stimulating preexisting cardiomyocyte proliferation by activating cell cycle, have been attempted previously to induce heart regeneration, although only modest effects have been achieved to date. Adult cardiomyocytes need to undergo dedifferentiation first before proliferation, if not simultaneously. In fact, adult zebrafish heart regeneration is accomplished through both dedifferentiation and proliferation. Chromatin state and remodeling is often associated with numerous physiological or pathological processes including organ development, aging, and cancer. However, it is unclear whether epigenetics dictates cardiomyocyte proliferation capacity, or, whether harnessing epigenetics in adult cardiomyocytes stimulates proliferation. Through a comparative transcriptomic analysis of murine embryonic day (E) 14.5 hearts (proliferation active) and adult hearts (proliferation inert), we identified a number of chromatin remodeling factors including histone deacetylase 7 (HDAC7) that are enriched in E14.5 hearts but missing in adult hearts. HDAC7 belongs to Class II HDACs, which have specific tissue distributions and shuttle between the nucleus and cytoplasm in response to signals. Studies have shown that knockout of HDAC7 compromises vascular integrity during heart development, while overexpression of HDAC7 induces tumor growth and epithelial proliferation. However, the potential role of HDAC7 in cardiomyocyte proliferation is undetermined. In our preliminary studies, upon knocking down of HDAC7 in cultured neonatal mouse cardiomyocytes (NMCMs), we found that cardiomyocyte proliferation was significantly decreased. By contrast, overexpression of HDAC7 in NMCMs resulted in significant cardiomyocyte dedifferentiation and increased proliferation. Further, overexpression of HDAC7 in adult cardiomyocytes in vivo significantly induced cardiomyocyte proliferation and improved cardiac function after myocardial infarction. Based on these novel and exciting preliminary findings, we hypothesize that HDAC7 is both necessary for cardiomyocyte proliferation and sufficient to reactivate postnatal cardiomyocyte proliferative and regenerative potentials. Three aims are proposed to test our central hypothesis. Aim 1: To determine the mechanisms by which HDAC7 promotes cardiomyocyte proliferation; Aim 2: To determine whether HDAC7 is required for cardiomyocyte proliferation; Aim 3: To test whether HDAC7 overexpression promotes adult cardiomyocyte proliferation and improves heart function after myocardial injuries. We intend to achieve these goals by using a synergistic approach of mouse genetics, developmental and molecular biology, and biochemistry. Results of these experiments will establish a novel and rigorous therapeutic strategy for promoting heart regeneration and pave a new path to effective heart repair in humans.